During the fabrication of transistors, resistors, diodes, and other microelectronic devices, a common practice is the implantation of a dopant into a substrate. In a transistor, the implant is directed to a portion of an active area between isolation regions. The dopant is typically B, P, or As ions that are implanted at high energy of up to 150 keV to form features such as lightly doped source/drain (S/D) regions or more heavily doped S/D regions in the substrate. Other parts of a device including a gate electrode and sidewall spacers adjacent to the gate may also be doped by an ion implant process.
The ion implant is performed through a mask that is usually a photoresist layer which is patterned to selectively expose regions of the substrate that are to be doped. The photoresist must be thick enough to prevent ions from reaching protected substrate regions. Unfortunately, the mask is not inert toward the high energy ions. As a result, the top portion of the photoresist layer is transformed into a hard carbonized crust that is difficult to remove. A plasma etch can strip the crust but often attacks the substrate or other portions of a device to cause a loss in performance.